Fluid used in machinery and the like (engines, compressors, hydraulic devices) can be monitored by infrared spectroscopy using known systems. In many cases, fluid is withdrawn from the machine and sent to a laboratory to determine the percentages of water, fuel, antifreeze, or the like in the fluid as a measure of machine health.
For in-situ systems, the fluid must be interrogated optically with at least the return radiation directed in some fashion to the infrared spectrometer. Generally, doing so involves using a waveguide (such as a fiber optic) to gather the radiation from the analysis cell, which resides within the fluid stream. See, for example, U.S. Pat. No. 5,170,056 incorporated herein by this reference. The analysis cell itself typically consists of either an attenuated total reflectance (ATR) element or a mirror coupled to the end of a fiber optic probe. Many such infrared spectroscopy systems for in-situ monitoring are complex and expensive. In place of a spectrometer-based system are systems such as in U.S. Pat. No. 8,213,006 (incorporated herein by this reference) where specific features of the fluid are desired and a filter or plurality of specially designed filters may be used to monitor such features. Such approaches may significantly decrease costs and increase optical throughput in systems where an entire infrared spectrum is not needed.